This is the basic and conceptual question from Chapter name- Arithmetic Progression
Chapter number- 9
Exercise 9.6
In this question we have been given that In an A.P. the sum of first n terms is 13n²/2 +. 13n/2
Now we have to Find its 25th term
CBSE DHANPAT RAI publications
Class:- 10th
Solutions of CBSE Mathematics
Question 47
Given Sn = 3n2/2 + 13n/2,
On putting n = 1, we get the first term(a), S1 = a = 3(1)2/2 + 13(1)/2 = (3 + 13) / 2 = 8
On putting n = 2 gives S2 = a + a + d = 3(2)2/2+ 13(2)/2 = 19
=> d = 19 – 2a
=> d = 19 – 16 = 3
The 25th term of the A.P., a25 = a + (25 – 1)d
= 8 + (24)3
= 6 + 72
= 80
Hence, the 25th term of the given A.P. is 80.